In food container laminates, aluminum foils have traditionally been used as sealing layers to protect the foodstuff from deterioration caused by contact with oxygen and moisture. Nowadays, the aluminum foils are to an increasing extent being replaced by various polymer films, which have good barrier properties, but which are more easily degradable in nature than aluminum. Since no single polymer can provide the same resistance to gas and water penetration as aluminum, the polymer-based laminates typically comprise multilayered polymer structures. Often, these structures incorporate thermoplastic polymers, such as polyolefins, which are heat sealable and thus make it possible to manufacture continuous structures. The polyolefins themselves are known to have excellent moisture and water vapor resistance, but they suffer from poor gas barrier properties. Therefore, the multilayered polymer structures usually include a specific gas barrier material, such as ethylene/vinyl alcohol (EVOH), a conventional polyethylene-EVOH-based multi-layer product having the following structure: PE/adhesive polymer/EVOH/adhesive polymer/PE).
It is known that thermotropic liquid crystalline polymers (LCP), have excellent barrier properties. However, the processing of them is difficult due to anisotropic behavior. Because the price of the liquid crystalline polymers is also rather high, LCP's have not been used as barrier materials in practical applications.
However, some LCP-based barrier structures are described in the prior art. Thus, EP Patent Application No. 0 503 063 discloses a composite film comprising a liquid-crystal polymer layer containing a thermotropic liquid crystal polymer and a thermoplastic polymer layer laminated on at least one surface of the liquid crystal polymer layer. The polymer layer is fixed on the liquid crystalline polymer through an adhesive layer. The use of such a layer can be avoided by functionalizing the thermoplastic in order to ensure proper attachment to the LCP film.
Similar structures are also described in JP Published Patent Applications Nos. 2 220 821, 2 253 949-2 253 951, and 2 261 456.
U.S. Pat. No. 5,084,352 describes a multilayered barrier film product, which includes a first polymer having low permeability to moisture and a heterogeneous polymer blend film containing a gas barrier polymer. The heterogeneous film comprises a barrier polymer, such as EVOH, and a second polymer, distributed within the barrier polymer. The second polymer consists of a polyolefin which is functionalized, so as to allow the heterogeneous polymer film to adhere to the first polymer film having low permeability to moisture. As an example of alternative barrier polymers, U.S. Pat. No. 5,084,352 also mentions thermotropic liquid crystal polymers. These polymers are, however, not suggested for use in packaging applications, nor are there any examples given on the actual use of a LC polymer in the described multilayered film product.
The main problem associated with the known barrier structures based on liquid crystalline polymers as a barrier to oxygen penetration resides in the high price of the LC polymer and the difficult processing of the LCP layer. Since only homogeneous LC layers have so far been used, the amount of the liquid crystalline polymer in relation to the other components of the polymer structure becomes rather high, which increases the total costs of the barrier structure. Furthermore, homogeneous LCP layers are brittle and difficult to process with traditional lamination techniques.